1. Field of the Invention
This invention relates to invasive medical devices. More particularly, this invention relates to monitoring of electrical activity within the heart using an invasive probe.
2. Description of the Related Art
Cardiac arrhythmia, such as atrial fibrillation, occurs when regions of cardiac tissue abnormally conduct electric signals to adjacent tissue, thereby disrupting the normal cardiac cycle and causing asynchronous rhythm.
Procedures for treating arrhythmia include surgically disrupting the origin of the signals causing the arrhythmia, as well as disrupting the conducting pathway for such signals. By selectively ablating cardiac tissue by application of energy via a catheter, it is sometimes possible to cease or modify the propagation of unwanted electrical signals from one portion of the heart to another. The ablation process destroys the unwanted electrical pathways by formation of non-conducting lesions.
Mapping of electrical potentials in the heart is now commonly performed, using cardiac catheters comprising electrophysiological sensors for mapping the electrical activity of the heart. Typically, time-varying electrical potentials in the endocardium are sensed and recorded as a function of position inside the heart, and then used to map a local electrogram or local activation time. Activation time differs from point to point in the endocardium due to the time required for conduction of electrical impulses through the heart muscle. The direction of this electrical conduction at any point in the heart is conventionally represented by an activation vector, which is normal to an isoelectric activation front, both of which may be derived from a map of activation time. The rate of propagation of the activation front through any point in the endocardium may be represented as a velocity vector.
Mapping the activation front and conduction fields aids the physician in identifying and diagnosing abnormalities, such as ventricular and atrial tachycardia and ventricular and atrial fibrillation, that result from areas of impaired electrical propagation in the heart tissue.
Localized defects in the heart's conduction of activation signals may be identified by observing phenomena such as multiple activation fronts, abnormal concentrations of activation vectors, or changes in the velocity vector or deviation of the vector from normal values. Examples of such defects include re-entrant areas, which may be associated with signal patterns known as complex fractionated electrograms. Once a defect is located by such mapping, it may be ablated (if it is functioning abnormally) or otherwise treated so as to restore the normal function of the heart insofar as is possible.
Mapping of the electrical activation time in the heart muscle requires that the location of the sensor within the heart be known at the time of each measurement. In the past, such mapping was performed using a single movable electrode sensor inside the heart, which sensor measured activation time relative to a fixed external reference electrode. This technique, however, requires calibration, for example impedance calibrations with adjustments for impedance unrelated to that of the body. Mapping of electrical activation time using a single electrode was, furthermore, a lengthy procedure, which was generally performed under fluoroscopic imaging, thereby exposing the patient to undesirable ionizing radiation. Further, in an arrhythmic heart, activation times at a single location may change between consecutive beats.
Because of the drawbacks of single-electrode mapping, a number of inventors have taught the use of multiple electrodes to measure electrical potentials simultaneously at different locations in the endocardium, thereby allowing activation time to be mapped more rapidly and conveniently, as described.
For example, commonly assigned U.S. Pat. No. 6,961,602, which is herein incorporated by reference, describes a catheter comprises a plurality of spines each capable of obtaining electrical, mechanical and locational data. The catheter comprises a mapping assembly having at least two spines, each having a proximal end attached at the distal end of the catheter body and a free distal end. Each spine comprises at least one location sensor and at least one electrode, preferably a tip electrode and at least one ring electrode. The spines may be arranged in an expanded arrangement wherein each spine extends radially outwardly from the catheter body or in a collapsed arrangement wherein each spine is disposed generally along the longitudinal axis of the catheter body. In use, at least one electrode from each spine is positioned in contact with heart tissue to map the electrical activity of the heart. The location sensors are used to determine the location of each point where the electrical activity is monitored.